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Acute And Chronic Meningitis Health Article

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Diagnosis

Cerebrospinal fluid examination

Examination of CSF is of paramount importance for the diagnosis of all forms of meningitis (Table 22.8). Accordingly, a lumbar puncture should be performed in patients who have suspected meningitis once a mass lesion that may lead to cerebral herniation has been ruled out on clinical grounds or by CT scan of the head. Evidence for mass lesions consists of the combination of focal neurologic signs (focal seizures, sensorimotor and visual defects) and clinical evidence of increased intracranial pressure (headache, vomiting, impaired mental status, papilledema). In patients who have suspected bacterial meningitis who show no evidence for a focal lesion or increased intracranial pressure, imaging studies before lumbar puncture are usually not necessary. While performing the lumbar puncture, CSF pressure should be recorded.

Immediate examination of the CSF provides valuable information. A Gram stain of uncentrifuged CSF (if the CSF is turbid) or centrifuged CSF (if it is not) indicates the presence of white blood cells, their approximate differential count (mononuclear versus polymorphonuclear) and whether bacteria are present.

Cultures for bacteria and fungi should always be performed, even in patients already treated with antibiotics. Tests for the detection of bacterial antigens by immunologic methods, such as latex particle agglutination, have sensitivities in the range of the Gram stain or culture and are of doubtful benefit when used routinely, but sometimes identify organisms in patients with partially treated bacterial meningitis and negative Gram stain and culture.

Cerebrospinal fluid concentrations of protein, glucose and lactate, in addition to the number and type of white blood cells are helpful in the differential diagnosis of various forms of meningitis (see Table 22.8). ⁶⁹

In addition to the routine tests carried out on all CSF samples, specific tests may be indicated under special circumstances. Viral cultures are mostly negative in CSF from patients who have aseptic meningitis and should not be performed routinely. However, nucleic acid detection by polymerase chain reaction and related tests is highly sensitive for detecting selected pathogens, particularly herpes viruses such as herpes simplex virus and cytomegalovirus. Serologic tests of CSF can establish the etiology of cases of chronic meningitis (syphilis, coccidioidomycosis). Isolation of enteroviruses from throat washes or stool should be attempted because after the first 3 days of symptoms enteroviruses are recovered only from these sites.

Systemic parameters

Signs of inflammation are most pronounced in acute bacterial meningitis, and in some cases of chronic meningitis (e.g. tuberculosis, fungal meningitis). In bacterial meningitis, the differential blood count will frequently show a leukocytosis with a left shift. The erythrocyte sedimentation rate and other acute phase reactants are typically elevated.

Blood cultures should be performed in all patients who have suspected meningitis before initiation of antibiotic therapy. They:

yield the infecting organism in more than 60% of cases of acute bacterial meningitis;
should be sterile in aseptic meningitis; and
rarely reveal the organism in chronic meningitis ( Brucella spp., Nocardia spp., fungi).

Patients who have acute meningitis should be examined for evidence of electrolyte imbalance. Hyponatremia is common and may indicate dehydration (in which case the urine sodium concentration is low) or the syndrome of inappropriate antidiuretic hormone (SIADH), in which case the urine sodium concentration is high, an assessment that is important for making the correct choice of fluid substitution, particularly in children who have acute meningitis.

Evidence for disseminated intravascular coagulation is most commonly seen in patients who have meningococcal meningitis. Platelet count, prothrombin time, partial thromboplastin time, fibrinogen levels and fibrin d-dimers should be measured in patients who have suspected disseminated intravascular coagulation.

Imaging studies

In patients who have bacterial meningitis, the possibility of focal infections of the head (sinusitis, otitis media) must be considered, and in selected cases, an appropriate radiologic test (e.g. a CT scan of the head) may be necessary. A CT scan of the head is also indicated in patients who have suspected intracerebral mass lesions or parameningeal foci. Chest radiographs in patients who have acute bacterial meningitis may reveal pneumonia, and in patients who have chronic meningitis, it may show evidence of pulmonary involvement by tubercle bacilli or fungi.